hearing dog

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A dog trained to assist a deaf or hearing-impaired person by signaling the occurrence of certain sounds, such as a doorbell or fire alarm.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A dog trained to assist the deaf, alerting the deaf person to doorbells, smoke alarms etc.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun dog trained to assist the deaf by signaling the occurrence of certain sounds